Wait by default do you mean without this installed? If you have the same amount of delay in default (no custom ringtones installed) and with a custom ringtone then what's the problem (besides your ringtone being slow on the N900 in general).

Though if we're going by rings I've found that my N900 starts ringing (in both default and custom) after 2-3 rings heard from the test phone I call with. This means I can see the person's picture come up with the call response options. It seems for the first two rings though nothing happens on the N900 (I can use it as usual).